1. Field of the Invention
The present invention relates to fluidized bed coal combustion and, more particularly, to a reactor incorporating a nozzle injector formed by a continuous ash removal assembly.
2. Description of the Prior Art
The U.S. reserve of coal is about 3 trillion tons. Although the most abundant (80%) fossil fuel in Amercia is coal, the U.S. consumption pattern is quite a reversal of form in terms of utilization, with coal representing only 17%, oil and gas about 78%.
The demand for all the fossil fuels combined is expected to double by the year 2000, even with the increasing use of nuclear power. While the domestic supply of crude oil and natural gas is not likely to keep apace with the energy demand, coal can play an important role in filling such a gap and thus reduce the requirements for imported supplies of oil and gas.
Coal, the fossilized plant life of prehistoric times, contains various amounts of sulfur due to the nature of its origin. Under most existing commercial technology, the generation of electricity from coal poses environmental problems because of sulfur oxides and particulate emissions. Since most of the coals in this country, particularly the Eastern and Midwestern coals, have high sulfur content (&gt;2%) there is a need for an economical process of converting high sulfur coals to electrical energy (&lt;1.2 lbs of SO.sub.2 emission per million BTU thermal output by EPA standard) without causing serious air pollution. If the vast coal reserve can be cleanly converted, it can supply most of the energy needs of the United States for the next three centuries.
At the present time, about one-half of the electric power in the United States is generated from natural gas and petroleum; most of the other half is from coal. If coal can be utilized for electric utilities, petroleum and natural gas would be released for other essential uses, especially as a starting material for the synthetic rubber and plastics industry.
Because of the critical domestic shortages in petroleum and natural gas, it is imperative that methods be developed to burn coal directly in an environmentally acceptable manner, and with maximum thermal efficiency. For the near term, however, there is a need for conventional coal-fired systems, particularly pulverized coal-fired (PCF) steam generators, and for rapid development of large pressurized, high temperature fluidized-bed reactors.
Fluidized bed combustion of coal differs significantly from conventional modes of firing coal such as mechanical stokers, PCF and cyclone. Crushed and sized coal is burnt in a bed of ash and lime or dolomite (sulfur scavenger) that is fluidized with air. This system utilizing high pressure and temperature has potential for more efficient and reliable boilers and for low sulfur oxide and nitrogen oxide emissions.
It should be noted that coal feedstocks may contain up to 11 wt.% of ash, which when combined with the ash generated during combustion as well as the ash from the added limestone, contributes to a rapid build-up which cannot be readily dumped from the high pressure vessel conventional fluidized bed reactors.
Another problem in addition to ash removal with conventional fluidized bed coal reactors is the containment of coal particles in the proper combustion zone. The known prior art fluidized bed systems often have coal combustion and particle ignition adjacent to the porous inlet grid. The hottest portion of the flame is often too far from the boiler tubes for efficient utilization of the thermal values.